The present invention relates to a power distribution system, and more particularly to an aircraft power distribution system which employs communication and load control between an aircraft electrical load control system and an Auxiliary Power Unit (APU).
Conventional gas turbine engines provide a variety of power to an aircraft in addition to propulsive thrust. The three main power draws from the engine are electrical, bleed air, and pneumatic/hydraulic. In addition to the engine driven generators that provide electrical power to the aircraft, bleed air is drawn off the engine and used for the aircraft pressurization, cooling and heating systems. The pneumatic/hydraulic systems are pressurized with pumps mechanically driven by an engine driven gearbox. Each of these conventional power distributions systems reduces the engine efficiency and the resulting thrust for aircraft propulsion. Such conventional power distribution systems are effective for current generation aircraft as such aircraft efficiently utilize the extensive bleed air, pneumatic, and mechanical power distribution systems which minimizes electrical power distribution system requirements.
Conventional power distribution systems utilize an Auxiliary Power Unit (APU) type gas turbine engine to provide supplemental electrical power to that provided by generators powered by the propulsion engines. The electrical power provided by a conventional APU type gas turbine engine is typically a small fractional part of the APU total load carrying capacity while the relatively larger fractional part is provided as bleed air, pneumatic mechanical output to the conventional power distribution systems.
Generally, the APU driven generator is the electrical load limiting system component. Further, electrical load is given priority as the APU controller avoids overload conditions by modulating the discretionary non-electric load. The APU controller on this type of combined pneumatic, mechanical, and electrical power APU system is autonomously capable of controlling the load.
Recently, aircraft systems are tending toward usage of electrically powered equipment eliminating the bleed air system. These “more electric” aircraft power distribution systems operate at significantly increased electrical power levels on the order of 1,000 kVA. Conditioning systems such as cabin pressure, cooling and heating are powered by electric motors. The hydraulic pumps are also driven by electric motors. In addition, without a bleed air system to spin-up the engine during start, the generator operates as a motor during the start sequence to spin up the turbine.
With the advent of the “more electrical” aircraft, APU load control is no longer possible solely within the APU system. Some load management shall require action of the aircraft electrical load system. As altitude increases, the APU load carrying capacity inversely decreases as a function of the reduced APU inlet pressure. Therefore as operation occurs at increasing altitudes the available power delivery capability decreases. The load management system for a “more electric” aircraft must shed discretionary electrical loads to protect the APU from operating at excessive gas path temperature conditions while continuing to provide required levels of electrical power associated with a specific altitude.
Accordingly, it is desirable to provide an aircraft power distribution system which efficiently facilitates load management in a “more electric” aircraft power systems.